It is well-known that RF devices operating in quasi-linear modes require biasing circuitry. In these modes, effective biasing circuitry should supply the RF device with its current requirements while accounting for drifts in the RF device's operating point due to temperature variations. These changes in operating point typically result in degraded amplifier performance and, furthermore, if uncontrolled, the drifting due to temperature may lead to catastrophic device failure (i.e., "thermal runaway").
Thus, the effectiveness of a biasing device is related to its current supply capability, its internal resistance (which also affects it current supply capability) and its ability to prevent thermal runaway (caused by uncontrolled drifting of the operating point). Additional considerations for determining the usefulness and appeal of a biasing device include the amount of space it consumes, the need for support devices, and the effects on total amplifier efficiency, linearity, stability and reliability.
Traditional biasing techniques such as resistive voltage division, diode bias or bypassed emitter bias can no longer satisfy the aggressive specifications imposed on RF transistors without substantial reduction in total system efficiency and/or common-emitter power gain.